Not tuning, but I've recently had the front silencer on mine replaced with a straight pipe. It does make a difference. Turbo spools up quicker and a little more grunt at low revs. Cheap too, at your nearest exhaust centre.
...and it sounds great too, lekker deep tone added to the exhaust without being any noisier (not that you could get much louder than a tdi at full chatter). In fact, I'm probably going to remove the back box as well.